Ban By Password?
 

Maybe a way to ban those users who use proxies and create new accounts easily;

Ban By Password
- Checks all other users for the same password, reports as necesary
- Bans the user's password; no accounts can be registered with the banned password, and when a user tries to create an account with the Banned password (maybe have this as a extra ban mode), they get IP banned

I'd like to see it. I bet most of the time, they use the same passwords, and will not think to change it.

nope, not possible, the salt removes the ability to compare MD5 hashes.